Integrated micromechanical sensor device and process for producing it

ABSTRACT

The integrated micromechanical sensor device contains a body with a substrate (1) on which an insulating layer (2) and thereon a monocrystalline silicon layer (3) are arranged, in which the silicon layer has trenches as far as the surface of the insulating layer, and the side walls of the trenches as well as the side of the silicon layer adjacent to the insulating layer have a first doping type (n + ) and the silicon layer has a second doping type (n - ) at least in a partial region of its remaining surface, in which the silicon layer has a transistor arrangement in a first region (TB) and a sensor arrangement in a second region (SB), for which the insulating layer (2) is partly removed under the second region. Such a sensor device has considerable advantages over known devices with regard to its properties and its production process.

This is a division of application Ser. No. 08/619,735 filed on Jun. 12,1996, now U.S. Pat. No. 5,744,719, which is a 371 of PCT/DE94/01092filed Sep. 20, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an integrated micromechanical sensor device andto a process for its production.

Micromechanical sensors are becoming increasingly established in allfields of technology, for example in navigation systems and motorvehicles, in particular in conjunction with safety systems. A largeproportion of such sensors form pressure and acceleration sensors. Thereis a requirement for reliable and compact sensors which are easy toproduce and therefore inexpensive, with high measurement accuracy andgood proportionality between the measured quantity and the outputsignal.

2. Description of the Related Art

Most pressure or acceleration sensors currently used are produced usinghigh-precision mechanical methods or by using KOH etching technology ona silicon base (bulk micromachining). The sensor signal, to date usuallyproduced by the piezoelectric effect, is evaluated separately from thesensor. There is, however, a trend toward intelligent sensors in whichthe sensor as well as the circuit for evaluating the sensor signal, andoptionally a test circuit, are integrated on a chip on the basis ofplanar silicon technology. Evaluation of the piezoresistive orcapacitive sensor signal, and linearization and amplification, takeplace using semiconductor circuits of known technologies. Such a sensoris, for example, disclosed by the published article F. Goodenough:Airbags Boom When IC Accelerometer Sees 50 G, Electronic Design, Aug. 8,1991, pp. 45-56.

Whereas conventionally produced micromechanical sensors are relativelylarge, expensive and inaccurate, the abovementioned published articledescribes an improved embodiment. The production of this known so-calledsurface micromechanical (surface micromachining) sensor requires, asemerges in particular from the related further published article: AnalogDevices Combine Micromachining and BICMOS, Semiconductor International,Oct. 1991, 21 masks, namely 6 masks for the sensor process and 15 masksfor a 4-μm BICMOS process. The comb-shaped sensor element for formingthe capacitive sensor consists of a 2 μm thick polysilicon element andis connected to the substrate surface via springs, which are also madeof polysilicon.

The production process for the known sensor is extraordinarily elaborateand expensive. Moreover, it is not certain that the polysilicon layersused for the mechanically moved parts of the sensor have sufficientlong-term mechanical stability. In addition to this possible degradationover time, the mechanical properties such as the elastic modulus orintrinsic stress of polysilicon is sensitively dependent on therespective process conditions during production. The thermal annealingof the intrinsic stress requires additional heat-treatment steps in theproduction process, which has a detrimental effect on the electroniccircuit simultaneously integrated in the sensor. Further to this,additional depositions of semiconductor layers are required in theproduction process. In one conceivable use of modern sub-μm-BICMOScircuits for the evaluation circuit of the sensor, the low processtemperatures then used make it no longer possible to produce stress-freepolysilicon layers.

DE-A-43 09 917, published after the priority date of the presentapplication, describes the use of a monocrystalline silicon layer withan overlying silicon nitride layer.

SUMMARY OF THE INVENTION

The invention provides an integrated micromechanical sensor device, inwhich a body is formed with a substrate, with a monocrystalline siliconlayer arranged thereon and with an insulating layer arranged in apredetermined region between the two, in which the silicon layer hastrenches from its surface to its lower boundary, in which, in thesilicon layer, the side walls of the trenches and the region of thesilicon layer associated with the lower boundary of the silicon layerhave a first predetermined doping type, and the silicon layer has asecond predetermined doping type at least in a partial region, in whicha transistor arrangement is provided in a first region of the siliconlayer, above the insulating layer, and in which a sensor arrangement isprovided in a second region of the silicon layer, in at least some ofwhich no insulating layer is present.

A process for the production of an integrated micromechanical sensordevice provides the following steps:

forming a body with an insulating layer arranged on a substrate, and amonocrystalline silicon layer arranged thereon, the silicon layer havinga predetermined doping type,

etching trenches in the silicon layer as far as the surface of theinsulating layer,

doping the trench walls,

producing a transistor arrangement in a first region of the siliconlayer, and

removing the insulating layer under a second region of the siliconlayer.

After the trench walls have been doped, the trenches can be filled withan insulating oxide layer. However, after they have been produced, thetrenches are preferably filled with a doping oxide (doping insulantlayer) which acts as a doping source for the subsequent doping of thetrench walls. The oxide in the trenches is likewise removed under thesecond region of the silicon layer, in conjunction with the insulatinglayer.

The sensor produced according to the invention contains monocrystallinesilicon. It avoids the use of polysilicon layers for the mechanicallymoved parts for instance monocrystalline silicon is used for theseparts. Monocrystalline silicon has accurately known mechanicalproperties which do not depend on the respective parameters of theproduction process. In addition, the mechanical properties are notsubjected to degradation over time, so that the long-term stability isvery great.

The invention has the further advantage that, with the use of known andavailable trench etching and filling processes, that it is fullycompatible with VLSI. Further, the sensor device according to theinvention is mechanically robust, since the moving parts are located inthe silicon layer and not on the chip surface. Since the electrodes ofthe sensor extend perpendicularly to the chip surface, a high specificcapacity (packing density) of the sensor is produced. At the same time,the sticking problem, that is to say adhesion of surfaces during orafter a rinsing process, is mitigated since the stiffness of the sensordevice perpendicularly to the oscillation direction is very great.

Finally, the sensor device according to the invention affords theconsiderable advantage that, when a bipolar or BICMOS process is used,the number of masks for producing the sensor device is not increasedcompared to a standard process in these technologies.

Developments of the invention are characterized in subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with the aid of figuresof the drawing, in which FIGS. 1 to 4, 5a, 5b and 5c show cross-sectionsthrough a device according to the invention in various steps of theproduction process, and

FIG. 6 shows a plan view of a capacitive sensor arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a base body 10 formed during the production of theintegrated micromechanical sensor device. An insulating layer 2 isarranged on a substrate 1 and a monocrystalline silicon layer 3 isarranged on this insulating layer. The substrate may likewise consist ofsilicon. Typically, the thickness of the insulating layer 2 is chosenbetween 0.5 and 1 μm, while the thickness of the silicon layer 3 may,for example, be between 5 and 20 μm. The crystal orientation and dopingtype of the substrate is arbitrary. The orientation and doping type ofthe silicon layer 3 corresponds to the basic technology used in theproduction of the sensor device and its semiconductor circuitarrangement.

In the illustrative embodiment according to FIG. 1 the side of thesilicon layer 3 adjacent to the insulating layer 2 or to the lowerboundary of the silicon layer is n⁺ -doped, while the surface region ofthe silicon layer remote from the insulating layer 2 is n⁻ -doped. Thedoping of the silicon layer 3 is not necessary for the actual sensorelement but exclusively corresponds to the technology which is to beused for the integrated circuit arrangement.

The base body according to FIG. 1 may, for example, be a DWB wafer, DWBstanding for Direct Wafer Bonding. Such wafers are formed by bondingtogether two semiconductor wafers and can be purchased on the marketwith high quality with the layer thicknesses and doping types shown inFIG. 1. A further possibility for the production of the base body shownin FIG. 1 is the use of the so-called SIMOX process (I. Ruge, H. Mader:Halbleiter-Technologie [semiconductor technology], Springer-Verlag, 3rdedition, 1991, page 237). In this case a silicon oxide insulating layeris formed by deep ion implantation of oxygen atoms into monocrystallinesilicon. This is followed by an epitaxy step. A third possibility forthe production of the base body according to FIG. 1 makes use ofrecrystallization, in which a silicon layer initially deposited inamorphous or polycrystalline form on a monocrystalline silicon layer anda silicon oxide layer arranged thereon is recrystallized by melting witha laser beam.

In a subsequent step, trenches are etched into the monocrystallinesilicon layer 3, as far as the surface of the insulating layer 2, forexample using an isotropic dry etching. The trenches are then filledwith a doping insulant. In the illustrative embodiment according to FIG.2, it is possible to use phosphorus glass (PSG) or boron-phosphorusglass (BPSG). The production of such glass layers is disclosed, forexample, by D. Widmann, H. Mader, H. Friedrich: Technologiehochintegrierter Schaltungen [VLSI circuit technology], Springer-Verlag,1988, pages 80 et seq. In a corresponding heat treatment, phosphorusand, optionally boron diffuse out of the phosphorus glass into thesilicon of the trench walls of the silicon layer 3. This produces thestructure shown in FIG. 2, in which the insulating layer 2 and thedoping insulant layer 4 adjoin at the bottom of the previously etchedtrenches. The trench walls are doped according to the region of thesilicon layer 3 adjoining the insulating layer 2, that is to say, in theillustrative embodiment, n⁺ -doped.

By the etching of trenches in the silicon layer and the doping of thetrench walls, both the region SB, in which the actual sensor isprovided, and the region TB, in which the electronic evaluation circuit,and at least one transistor arrangement for the processing of the sensorsignal, is provided, are structured and isolated from each other. Theregion TB contains one or more insulated wells into which CMOS, bipolaror other components are incorporated, according to requirements. If thetransistor arrangement to be produced in the region TB is, for example,a bipolar transistor, with the structure of the region TB shown in FIG.2, a buried collector region and a low-impedence collector terminal inthe form of the doped trench walls are already produced. In contrast toarrangements in the prior art, the production of the structure shown inFIG. 2 does not require separate masking and doping processes for theburied region, for a channel stopper and for a collector. An epitaxyprocess can likewise be omitted.

Starting from the structure in FIG. 2, a transistor arrangement is thenproduced in the region TB. This transistor arrangement can be producedusing a standard bipolar or BICMOS process. Examples of such processesare, for example, disclosed by the abovementioned publication byWidmann/Mader/Friedrich: Technologie hochintegrierter Schaltungen. Inthe case of a bipolar transistor structure, starting from FIG. 2, thebase region may, for example, be produced first, and in a BICMOSprocess, the p or n well may be produced first.

It is, of course, also possible, starting from the structure in FIG. 2,to produce a MOS transistor arrangement in the base body. In this casealso, the standard process starts with the production of a p or n wellin the region TB intended for the transistor structures.

During the production of the transistor arrangement, the region SB,which is intended for the sensor element, is covered by a correspondingmask. According to FIG. 3, by way of example, a bipolar transistor isproduced, the collector C of which is connected with low impedence viathe buried region BL and the heavily doped low-impedence trench walls CAto the collector terminal K. A p-doped base is connected to the baseterminal B. The emitter terminal E is accordingly arranged above theheavily doped n⁺⁺ region. The base, emitter and collection regions ofthe transistor are insulated on one another by insulation regions I1 toI3, preferably of silicon oxide SiO₂. A passivation layer P is thenapplied over the entire arrangement. By way of example, the passivationlayer may consist of silicon nitride Si₃ N₄.

Following the production of the transistor arrangement, the passivationlayer P is removed over the sensor region SB with the aid of a resistmask FM, and then first the doping insulant 4 in the trenches of thesensor region, and subsequently the insulating layer 2, are removed atleast in the partial regions of the sensor region SB. This can, forexample, by a wet chemical or dry etching process. The insulating layer2 is in this case removed completely under the mobile electrodes of thesensor element, and not completely under the nonmoving electrodes of thesensor element, so that the latter is still mechanically connected tothe substrate 1.

FIG. 4 shows an arrangement after the removal of the insulant 4 and ofthe insulating layer 2 near and under the moving electrodes BE1, BE2 ofthe sensor region SB, while the insulating layer 2 is still partiallypresent under the fixed electrodes FE1, FE2 and FE3. In the arrangementaccording to FIG. 4, the transistor region TB is structured differentlythan in FIG. 3. While nothing has changed as regards the dopingconditions of the silicon layer 3 in the arrangement in FIG. 4 comparedto FIG. 2, an oxide layer 5 has been provided toward the edge of thesensor region in the transistor region of FIG. 4. The transistor regionis covered by a passivation layer P and a resist mask FM lying above thelatter.

Subliming chemicals, for example cyclohexane or dichlorobenzene, may beused in order to solve a sticking problem possibly occurring during theremoval of the insulating layer 2 under the region of the mobileelectrodes BEi.

The mask FM is removed following the described removal of the insulatinglayer.

FIG. 5 shows cross-sections through a sensor structure which isrepresented in plan view in FIG. 6. It is an acceleration sensor whichhas a moving mass part M which is suspended by spring elements F1, F2and other spring elements (not shown) from the silicon layer 3. The masspart M has finger-shaped moving electrodes BE10, BE11 and BE1i whichproject freely into regions between fixed electrodes FE10 to FE13 andFE1i. Capacitors, which are represented in FIG. 6 and in FIG. 5a purelyschematically, are formed between the moving electrodes BEi and fixedelectrodes FEi since the trench walls of the electrodes are heavilydoped. FIG. 5a shows, for this case, a section along the line A--A, andFIG. 5b shows a section along the line B--B, these lines being indicatedin FIG. 6. In order, during the production of the sensor part, to makeit possible to reliably remove the insulating layer under the mass part,the mass part M contains holes L. In principle, however, the holes arenot strictly necessary. The region of the special mask used forproducing the sensor structures is sketched in FIG. 6. Within the regiondefined by the special mask SM, the doping insulant is completelyremoved from the trenches and the insulating layer near and below themoving electrodes, the mass part and the springs.

FIG. 5 shows the equivalent circuit diagram obtained with the sensorstructure of FIGS. 5a and b, and FIG. 6. The moving electrodes BEi, thatis to say BE10i to BE1i can be connected to a terminal BEA via the masspart M and the springs Fi. The fixed electrodes are connected in pairsto terminals AE and CE which correspond to the fixed plates of acapacitor. The moving electrodes form a moving capacitor plate, so thatFIG. 5c represents a differential capacitor.

The doping insulant can be removed from the intermediate spaces ZRbetween two fixed electrodes, FIG. 5a. It is, however, possible for theintermediate spaces ZR of the fixed electrodes and the underlyinginsulating layer, which is adjacent to the respective fixed electrodes,to remain. This requires a different mask, with which the insulatinglayer and the doping insulant below and between the moving electrodescan be removed.

As an alternative to the structure in FIG. 5 and FIG. 6, the fixedelectrodes can be mechanically connected to the substrate 1 or theinsulating layer 2 in similar fashion to the abovementioned publication"Electronic Design". Fastening via a support results in self-supportingelectrodes, which means that the fixed electrodes must have a flexuralstiffness with regard to the acceleration forces which are exerted onthem, which is sufficient to prevent additional measurement errors fromhaving a negative effect on the measurement accuracy.

The sensor arrangement represented in FIG. 6 reacts sensitively to themovements of the mass part in the arrow direction indicated. Thepermissible deflection of the finger-shaped capacitive moving electrodesis less than the size of the gap separating it from a fixed electrode,that is to say less than approximately 1 μm. For this reason, theevaluation circuit of the sensor device is preferably configured as acontrol circuit, in such a way that a control voltage opposes adeflection of the mass part, in the sense that the partial capacitancesformed by the differential capacitor are always equal. This method hasthe advantage that it is a centre-zero method and is therefore as a rulemore accurate than an absolute method for determining the capacitancechanges.

A two-dimensional acceleration measurement is possible if use is made oftwo sensor arrangements according to FIGS. 5 and 6 which are offsetrelative to each other by 90°. The oscillation directions of therespective mass parts are in this case two mutually orthogonaldirections in the oscillation direction of the chip plane. The describedtechnique can similarly be applied to the production of differentialpressure sensors.

The invention has the following advantages:

The sensor mass part, the electrodes and the structural bars, that is tosay the suspension springs of the sensor, consist of monosilicon, sothat for the moving parts the flexural distortions and stresses knownfrom polysilicon are avoided.

The mass part of the sensor and the spring constant of the flexural bar,as well as, in the case of a capacitive sensor, the sensor capacitance,can be set independently of one another so that sensor arrays can beproduced with ease. The sensor arrangement makes it possible for thesensor to have a high packing density, since the electrodes extendperpendicularly to the chip surface, so that a large capacitive area canbe achieved. The stiffness of the electrodes perpendicularly to theoscillation direction is very great, since the section modulus isproportional to the 3rd power of the electrode thickness. For thisreason, where applicable, no sticking problem occurs, so that chemicalsto counteract this problem are not required.

Since the moving parts of the sensor are located in the silicon or inthe silicon layer, and not on the chip surface, the sensor isextraordinarily mechanically robust. By virtue of the arrangement of theelectrodes and of the mass part in the chip plane, an overload cutout isalso automatically provided in the chip plane.

When a bipolar or BICMOS standard process is used as the basictechnology for the evaluation circuit of the intelligent sensor, thenumber of masks is not increased. This allows considerable cost savingsand overall simplification of the production process.

In principle, the process and sensor device according to the inventioncan be combined with all known technologies. In particular, the sensordevice is compatible with VLSI, so that structure dimensions of lessthan 1 μm can be achieved. The trench etching and filling processes, aswell as the usual semiconductor processes, known from semiconductortechnology can therefore be employed during production.

What is claimed is:
 1. A process for producing an integratedmicromechanical sensor device, which comprises:a) forming a body with aninsulating layer disposed on a substrate, and a monocrystalline siliconlayer disposed on the insulating layer, the silicon layer being of adoping type; b) etching trenches with trench walls into the siliconlayer and to a surface of the insulating layer; c) doping the trenchwalls; d) producing a transistor configuration in a first region of thesilicon layer; and e) removing the insulating layer below a secondregion of the silicon layer.
 2. The process according to claim 1, whichfurther comprises doping the silicon layer with a first doping type on aside thereof adjacent the insulating layer, and doping the silicon layerwith a second doping type on an opposite side thereof.
 3. The processaccording to claim 1, which further comprises filling the trenches witha doping insulant subsequently to step a), and exposing the trenches atthe second second region prior to step e).
 4. The process according toclaim 1, which further comprises filling the trenches with phosphorusglass subsequently to step a) and exposing the trenches at the secondregion prior to step e).
 5. The process according to claim 1, whereinstep c) comprises doping the trench walls in correspondence with thedoping type of the silicon layer.
 6. The process according to claim 1,wherein step d) comprises producing the transistor configurationaccording to one of bipolar, MOS or bipolar/MOS standard process.
 7. Theprocess according to claim 1, which further comprises, subsequently tostep d), removing a passivation layer in a sensor region, andsubsequently removing the insulant from the trenches of the sensorregion and the insulating layer below the silicon layer.